1. Field of the Invention
The present invention relates to a cooling system or apparatus in the field of liquid and gas chromatography instruments heated by radiant energy such as microwave or radiowave radiant energy and to methods for making and using same.
More particularly, the present invention relates to a cooling system or apparatus in the field of liquid and gas chromatography instruments heated by radiant energy such as microwave or radiowave radiant energy and to methods for making and using same. The cooling system includes a cooling apparatus having an coolant inlet and a coolant outlet, a supply of a coolant, a conduit connecting the cooling apparatus to the coolant supply, where the cooling system is adapted to permit lower start temperatures and faster post run cool down resulting in faster cycling between samples or shorter sample cycle times. The present invention also relates to a microwave oven for use in liquid and gas chromatography instruments including a microwave oven having such a cooling apparatus and to methods for making and using same.
2. Description of the Related Art
Gas and liquid chromatography are physical methods for the separation, identification, and quantification of chemical compounds. These methods are used extensively for applications that include the measurement of product purity in analytical chemistry, the determination of environmental contamination, the characterization of natural substances, and the development of pharmaceuticals.
The fundamental methods used in gas and liquid chromatography instruments to separate chemical constituents are similar. A sample mixture is injected into a flowing neutral carrier stream and the combination then flows through a tube or chromatographic column. The inner surface of the column is coated or the tube is packed with a material called the stationary phase. As the sample mixture and carrier stream flow through the column, the components within the mixture are retained by the stationary phase to a greater or lesser degree depending on the relative volatility (in the case of gas chromatography) or the relative solubility (in the case of liquid chromatography) of the individual components and/or on their respective affinities for the stationary phase. When the individual mixture components are released into the carrier stream by the stationary phase, they are swept towards the column outlet. As the combined flow exits the column outlet, the flow is forwarded to a detector, where the separated components are detected and measured. Different chemical components or compounds in the sample are retained for different times by the stationary phase or spend different amounts of time in the moving phase. By measuring the retention times, the specific compounds or components in the sample can be identified. The relative concentration of the components compounds is determined by comparing peak amplitudes measured with the detector for each compound or component in the sample. The peak amplitude are generally compared to peak heights for that component of know concentrations of the component or are derived from instrument calibration.
The primary difference between gas chromatography (GC) and liquid chromatography (LC) is the mode of separation. In gas chromatography, the sample is volatilized and propelled down the analytical column by a moving stream of gas. In liquid chromatography, the sample is dissolved and propelled down the analytical column in a moving stream of liquid. Another difference between gas and liquid chromatography is that the columns used in liquid chromatography have stationary phases that fill or are packed into the tube; while those used in gas chromatography can be packed, but generally the stationary phase is coated or bonded to the interior wall, instead.
GC and LC measurements are facilitated by the application of heat to the chromatographic column to change its temperature. The use of a heated column oven in gas chromatographic systems greatly increases the number of compounds that can be analyzed and speeds up the time required for each analysis by increasing the volatility of higher molecular weight compounds. Heating an LC column affects the relative solubility of the mixture's components in the two phases and can enhance the separation as well as improve the repeatability of the elution times of the component chemicals.
Many methods have been described for heating chromatographic columns. The simplest and most commonly used method utilizes resistive heating elements to heat air which is in turn circulated through an insulated oven in which the column is placed. For example, U.S. Pat. No. 3,527,567 to Philyaw et al. describes a GC oven heated with resistive elements.
The resistive element heating method has several limitations. To achieve even heating of the column, a large volume of air is rapidly circulated around the chromatographic column. In addition to heating the column, the air heats the oven itself. Because the thermal mass of the oven is much larger than that of the column, the rate at which the column can be heated is commensurately reduced. A related problem is cooling time. After heating the oven to a high temperature during an analysis, it takes significantly longer to cool the oven plus the column to their initial temperature so that the next sample may be analyzed than it would to cool the column alone. Together, these limitations reduce the throughput of the chromatography instruments.
Attempts to localize the resistive heat element onto the column itself so as to reduce or eliminate peripheral heating of the “oven” are described in U.S. Pat. No. 3,169,389 to Green et al., U.S. Pat. No. 3,232,093 to Burow et al., and in U.S. Pat. No. 5,005,399 to Holtzclaw et al. Each of these patents describe methods for directly wrapping or cladding the chromatographic column with a resistive heating element. Methods are also described for positioning the resulting metal clad column adjacent to a cooling source to decrease cooling times. This method of heating can be difficult to implement in practice because of uneven heating of the column due to local hot or cold spots in the resistive heating element surrounding the column. Uneven heating of the column in turn compromises the quality of the analysis.
Yet another limitation of all resistively heated chromatographic devices is that if operated improperly, they can be driven to temperatures higher than the maximum tolerated by a given column resulting in damage to or destruction of the column.
An alternative method for heating chromatographic columns is microwave heating as described in U.S. Pat. No. 4,204,423 or radio frequency heating described in U.S. Pat. No. 3,023,835. Additional background information on microwave heating instruments can be found in U.S. Pat. Nos. 6,514,316, 6,316,759, 6,182,504, 6,093,921, 6,029,498, and 5,939,614, incorporated herein by reference.
Although the microwave heated chromatography instruments have been disclosed, these units are not well equipped for low temperature starts or for fast sample cycling. Thus, there is a need in the art for microwave heated chromatography instruments including a cooling apparatus that permits lower temperatures starts and faster sample cycling, i.e., reduced time between sample injections.